The presence of ortho isomers of dialkyl substituted benzenes in isomer mixes of these compounds is known to be undesirable. Ortho-isomers of dialkyl substituted benzenes are to be avoided because upon dehydrogenation of such materials to form vinyl aromatic products ring closures can occur and form bicyclo derivatives such as indenes and indanes. The latter compounds both adversely affect the properties of the desired vinyl aromatic products and reduce the recoverable amount of such products from the dialkyl substituted benzene isomer mix. If such compounds are formed separation by expensive superfractionation processes prior to dehydrogenation is required. However, thermodynamics predict that a substantial amount of ortho isomer will be present in a dialkyl substituted benzene product mix of para, meta, and ortho isomers. For example, a thermodynamic equilibrium for an isomer mix of ethyltoluene is approximately 31.5% para, 50.2% meta and 18.3% ortho at temperatures effective for vapor phase alkylation. Since the isomer mixes are only difficulty separated into para, meta, and ortho fractions, processes which produce a reduced amount of ortho in the original isomer mix are highly desirable.
Recently aluminosilicate type zeolite catalysts, including those known as "ZSM-5" type catalysts materials have been reported to be suitable for hydrocarbon conversion processes and, in particular, for the alkylation of aromatic substrates. One problem with these types of catalysts, however, is that they are subject to rapid deactivation in the presence of even small amounts of water. Thus, when using such catalysts, it is sometimes necessary to reduce the moisture content of feedstock materials prior to their introduction into a conversion zone. Furthermore to the extent such materials have been disclosed as being useful in isomer selective processes (such as the para-selective processes disclosed in U.S. Pat. No. 4,086,287, 4,117,024, 4,117,026, 4,127,616, 4,128,592 and the ortho suppressive process disclosed in U.S. Pat. No. 4,094,921) these aluminosilicates zeolites must be modified either chemically or by prior steam treatment or coking in order to be useful as isomer selective catalyst materials. Thus, a process for alkylating aromatic substrates to obtain dialkyl aromatics whereby production of the ortho isomer is suppressed by employing catalyst materials which do not require special modification and have a high level of conversion would be desirable.